This invention relates to the nuclear power reactor control art and has particular relationship to the load-follow operation of a nuclear reactor which constitutes the primary source of power supply apparatus. In the interest of facilitating the understanding of this invention by reference to a concrete embodiment of this invention, this application predominately deals with a light-water pressurized-water-reactor (PWR) power-supply apparatus. This invention is also applicable to apparatus of other types, for example, boiling-water reactor apparatus and any application of this invention to such other apparatus is within the scope of this application.
A PWR power supply apparatus includes a nuclear reactor having a nuclear fuel core bathed in a coolant which derives heat energy from the core and serves as primary fluid for the apparatus. Primary conductors are connected to the reactor for circulating the heated coolant typically from above the core through steam generators and then back to the lower part of the core. The primary coolant converts water (or other fluid) in the steam generator into steam (or other vapor) which is circulated, through secondary conductors, to drive a turbine. The vapor is then condensed and circulated back as feedwater to the generator.
It is necessary that under all conditions of loading of the turbine the reactivity of the core should be maintained such that for each neutron which produces a fission, one, and only one, neutron be generated. To achieve this purpose the reactor has a neutron-absorbing medium. This medium includes control rods and part-length (P/L) rods which are inserted and removed from the core, on changes in loading, in an effort to match the neutron-absorbing medium to the loading and at the same time to maintain the power distribution axially along the core as nearly uniform as practicable. In addition chemical shim is changed. The expression chemical shim means the injection of material into the coolant which may increase or reduce the neutron absorption. A typical chemical neutron absorbing component of chemical shim is boron (B.sup.10) which is added to the coolant as boric acid. The neutron absorption can be decreased by replacing the absorbing chemical by water which is also part of the chemical shim.
During turbine-load follow or other operating modes of a reactor requiring movement of the control rods into or out of the fuel core, the axial power distribution along the reactor may be shifted so that it is not within acceptable limits. The part-length (P/L) rods are then moved into or out of the core to correct this maldistribution. Chemical shim is also adjusted to add or remove neutron absorbing medium. In accordance with the teachings of the prior art the adjustment of chemical shim is effected by observing the position of the control rods and adjusting the shim so that the control rods may be kept within a desired well defined operating band. A well defined operating band is a band of movement of the control rods within which the maximum power-change capability, and particularly the spinning reserve, is preserved. The spinning reserve is the difference between any current operating power level of the apparatus and the power level which can be reached in the event of a sudden large demand of power. With the P/L rods moving into or out of the core, as well as the usual primary-to-secondary power mismatch which occurs, it is not possible to predetermine the well defined operating band of the control rods because they are responding to the reactivity variations produced by the movement of the P/L rods as well as the primary-to-secondary power mismatch.
U.S. Pat. Nos. 3,222,254 Maldague et al. and 3,578,562 Johnson et al. are typical of prior art teaching as to adjustment of the chemical shim. Maldague's teaching is to adjust the chemical shim so that the sum of the effects of the reactivity of the adjusted chemical shim and of the Xenon 135, a poison which is generated by the fission, remains constant. Johnson's teaching is to measure the neutron absorption properties of a portion of the coolant passed through a detector and to add or remove neutral absorbing medium by adjusting chemical shim in dependance upon the measurement. Neither Maldague nor Johnson deal with the problem of eliminating maldistribution while retaining the desired power output characteristic of the apparatus.
It is an object of this invention to overcome the difficulties and disadvantages of the prior art. Specifically it is an object of this invention to provide nuclear power-supply apparatus in which, by control of the chemical shim, the power distribution axially along the nuclear reactor core shall be maintained within acceptable limits while preserving a predetermined set of steam generator operating conditions and the maximum power-change capability and particularly the spinning reserve. It is also an object of this invention to provide a method for operating such apparatus.